Various drilling methods and systems are known in the art. Most arrangements use a rotating drill bit that is carried and conveyed in the wellbore by a drill string, which is in turn carried by a drilling rig located above the wellbore. The drill bit may be rotated by rotation of the drill string and/or the drill string may include as part of a bottom hole assembly a downhole rotary motor for rotating the drill bit.
Drilling fluid is pumped to the drill bit through the drill string and is directed out of nozzles in the drill bit for cooling the bit and removing formation cuttings. The drilling fluid may also provide hydraulic power to downhole tools, such as a mud motor located in a bottom hole assembly (BHA) for rotating the drill bit. Drilling fluid or mud may also provide hydraulic pressure in the wellbore to prevent collapse of the wellbore and/or fluid entry from the formation to the wellbore. The drilling fluid and any entrained formation cuttings are forced from the bottom of the wellbore by the continued pumping of drilling fluid through the drill string and then carried upwards through the annulus that exists between the drill string and the wellbore wall.
In cases of drilling offshore wells, the drilling rig may be positioned above the surface of the water, generally over a wellhead. A riser may be provided between the drilling rig and the wellbore at the seafloor for allowing the drill string to be conveniently run into and tripped out of the wellbore. The riser may also provide an extension of the annular wellbore flow path for returning the drilling fluid and cuttings to the rig for processing and/or reuse.
The wellhead may carry a blowout preventer (BOP) stack, which may include ram BOPs and/or an annular BOP, for example. BOPs may include an axial passage to accommodate the drill string and may include one or more closure devices, such as shear, blind or pipe rams or elastomeric packers to shut in the wellbore in the case of an emergency. A rotating control device (RCD), also sometimes referred to by routineers as a rotating control head, rotating blowout preventer, or rotating diverter, may be carried atop the BOP stack for preventing escape of well annulus fluid into the environment.